Cathodic glow discharge lamp



Nov. 23, 1965 J. E. WHlTE 3,219,867

CATHODIC GLOW DISCHARGE LAMP Filed May 51, 1962 1:: 27 mi g lnvervtov: do m E. Whi te b3 6 b His Ad,- OTTWSH United States Patent 3,219,867 CATHODIC GLOW DISCHARGE LAMP John E. White, Cleveland Heights, Ohio, assignor to General Electric Company, a corporation of New York Filed May 31, 1962, Ser. No. 198,921 7 Claims. (Cl. 313-178) This invention relates generally togaseous electric discharge devices, and more particularly to low voltage thermionic arc discharge lamps of the cathodic glow type. Such lamps generally operate at low voltages not much higher than the minimum voltage required to achieve ionization in the discharge medium.

Examples of the type of discharge lamp in which my invention is particularly useful are the lamps designated commercially RP'l2. and 2W-T6. These lamps are miniature single-ended fluorescent lamps utilizing a thermionic arc in low pressure mercury vapor to produce 2537 A. radiation which excites a longer -wave length ultraviolet emitting phosphor coated on the envelope wall. One of their fields of application is in aircraft or vehicle instrument panel lighting wherein the instrument dials or indicia are coated with a fluorescent material which produces visible light upon irradiation by the long wave ultraviolet or so-called black light from the lamp.

In order to make these lamps practical and economiically attractive for dashboard and instrument lighting in automotive vehicles using the now common l2-volt battery system, it is necessary that they start reliably at voltages at least as low as 11.5 volts. Desirably the lamp should start upon mere application of the battery voltage through the simple resistive ballasting circuit which limits or regulates the discharge current through the lamp, and without resort to special circuits such as inductive peak devices whose cost would make the system unattractive.

In my copending application Serial No. 173,407, filed February 15, 1962, entitled Electric Discharge Device and assigned to the same assignee as the present invention, I have described and claimed lamps of the present kind which start reliably at room temperature at voltages as low as 10.4 volts. These lamps make use of an auxiliary high work-function cathode to provide initial ionization; one form utilizes a low work-function main cathode consisting of a tungsten filament coated with alkaline earth metal oxides and an auxiliary high work-function cathode consisting of a bare tungsten wire. Although these lamps will start reliably under low batteryvoltage at room tem perature, starting is marginal and may become erratic when the condition of low battery voltage occurs simultaneously with low ambient temperature.

In copending application Serial No. 198,922, of Bentley T. Barnes, filed concurrently herewith, entitled Cathodic Glow Gaseous Discharge Device and assigned to the same assignee as the present invention, there is disclosed and claimed an improvement in lamps of the. present kind which lowers the ignition voltage below 11.5 volts, even in ambient temperatures as low as Fahrenheit. The improvement is achieved by admixing a minor'proportion of xenon to the argon in the buffer gas mixture, "a preferred mixture being 95% argon-5% xenon at a pressure of 5 millimeters of mercury. However such use of xenon 'in the buffer gas mixture decreases somewhat the radiant output or the efiiciency of the lamp. For some applications, it is desirable to be able to start at yet lower voltages; also a greater margin of safety to take care of product variability is desirable.

The object of the invention is to provide improvements in cathodic glow discharge lamps which will lower the ignition voltage at low ambient temperatures, desirably to less than 11.5 volts at a temperature-of zero degrees F., without appreciably affecting efiiciency or radiant output.

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In accordance with my invention, I have discovered that the ignition voltage at low temperatures can be reduced by generating a puff of mercury vapor in the discharge space. The puff of mercury vapor may be produced by action of the cathode or auxiliary cathode when current isfirst turned on, and starting then occurs during occurrence of the puff. With an oxide cathode and under appropriate conditions of cathode activation, the oxide cathode itself holds considerable mercury. Then when a cold lamp is turned on, the heating vaporizes mercury off the cathode, producing a vapor pufi which facilitates ignition. However starting in this manner is not always reliable since it depends upon prior condensation of mercury on the cathode which in turn is dependent upon the prior history or cycling of the lamp.

A preferred arrangement in accordance with my invention makes use of an amalgam-forming material like gold or a mercury absorbing material like charcoal with means for heating the material. During normal operation of the lamp, the amalgam comes into equilibrium with the mercury vapor at the pressure existing in the envelope, and when the lamp is turned off, the amalgam may absorb more mercury as a result of cooling faster than the envelope. At starting, the amalgam desirably heats up at a faster rate than the envelope and releases or vaporizes mercury, thereby producing the desired pufi. The amalgam may be heated directly by current flow through the body supporting it or indirectly by radiation or conduction from the neighboring cathode.

For further objects and advantages and for a better understanding of the invention, attention is now directed to the following description and accompanying drawing illustrating preferred embodiments. The features believed to be novel will be more particularly pointed out in the appended claims.

In the drawing wherein like reference numeral denote corresponding elements in the several figures:

FIG. 1 is a side elevation view, partly sectioned, of a miniature single-ended fluorescent lamp embodying the invention.

FIG. 2 is a schematic diagram of a circuit suitable for operating the lamp of FIG. 1.

FIG. 3 is a side elevation View of another lamp forming a preferred variant of the invention. I

FIG. 4 is a schematic diagram of a circuit suitable for the lamp of FIG. 3.

Referring to FIG. 1, the illustrated lamp 1 corresponds to the type designated commercially 2W-T6 having a nominal rating of 2 watts atan operating voltage of 11.5 volts. The envelope 2 of the lamp is generally tubular with a rouded end; typically it has a sealing length of 1%ths inches and a diameter of inch. The lamp contains an ionizable medium consisting of a rare gas or rare gas mixture serving as a buffer gas, for example argon or argon with up to 15% xenon at a pressure less than 10 millimeters of mercury, desirably at a pressure from 3 to 5 millimeters of mercury. A small quantity of mercury provides a pressure of mercury vapor determined by the operating temperature of the lamp but-generally less than microns. The envelope is coated internally with a phosphor 3 which fluoresces in the long wave ultraviolet region, for instance a cerium activated calcium phosphate or an aluminum silicate. Alternatively, if a source of short wave ultraviolet radiation is desired, the phosphor coating is omitted and a vitreous material which transmits 2537 A. radiation is used for the envelope.

The lamp is provided with an anode 4 mounted on an inlead 5, a main cathode 6 supported across inleads 7,8 and an auxiliary cathode 9 supported across inlead 7 which is common to the main cathode, and another inlead 10. The inleads extend through a conventional steam 11 sealed peripherally at 12 to the bulb and including an exhaust tube 13 which is tipped off at 14 after evacuation and gas-fill of the lamp. The anode 4 is preferably a strip of titanium metal bent to a U-shape, inverted, and surrounding the main cathode 6 with one leg extended downwardly. The main cathode is a triplecoiled tungsten filament coated over its central portion with electron emissive material of low work function such as alkaline earth oxide, for instance a triple mixture of barium, strontium and calcium oxides. Typically, the main cathode operates at a dull red heat, e.g. 800 to 1200 C., and supports a discharge current of approximately 200 milliamperes. The auxiliary cathode 9 may consists of a coil of bare tungsten wire or, alternatively, it may be a length of straight tungsten wire; it operates at white heat, e.g. 1800 to 2800 C., to support a discharge current of a few milliamperes at most. As explained in my aforementioned copending application, the auxiliary cathode has a high work function and emits a relatively small proportion of electrons having high potential energies. These higher potential energy electrons (as a result of the contact potential difference between main and auxiliary cathodes) are accelerated to higher velocities and generate enough ions to partially neutralize the electron space charge at the main or oxide coated cathode, thereby permitting increased current flow from the main cathode resulting in ignition of the thermionic arc. In a typical lamp, the U-shaped portion of the anode may be bent around a diameter of 3 to 4 millimeters, the main cathode core may have a diameter of 1 millimeter, and the auxiliary cathode may be a length of tungsten wire 1.3 mils in diameter and about millimeters long, located about 1.5 millimeter below and parallel to the main cathode coil.

A suitable circuit for energizing the lamp from a low voltage D.-C. supply is illustrated schematically in FIG. 2. The battery 15, a storage battery of 12 volts nominal rating, has its negative pole connected to inlead 7 which forms the common junction of main cathode 6 and auxiliary cathode 9. By closing switch 16, the positive pole of the battery is connected through current limiting resistance or ballast 17 to the anode 4; at the same time, a circuit is completed through resistor 18 to main cathode 6, and through resistor 19 to auxiliary cathode 9. Ballast resistance 17 is a tungsten filament lamp having a large positive temperature coeificient of resistance.

When switch 16 is first closed and before the lamp has ignited, only cathode heating current fiows through ballast lamp 17; the ballast lamp then is relatively cold and its filament resistance is low so that there is minimum reduction in the voltage available for ignition of lamp 1. After ignition, the current through the ballast lamp increases by the amount of the discharge current through lamp 1, so that its resistance increases appreciably. There is then a relatively constant voltage drop across lamp 1, and variations in battery voltage are countered by variations in voltage drop across ballast lamp 17.

For testing for are ignition voltage, a circuit similar to that of FIG. 2 may be used but including of course provision for varying the supply voltage and regulating the discharge current.

The lamp of FIG. 1 as thus far described is to a certain extent capable of generating a puff of mercury vapor to facilitate starting at low temperatures. When the oxide coated cathode is properly activated, the oxide coating itself holds considerable mercury. When the cold lamp is turned on, the cathode heats up and the mercury is vaporized ofr, thereby producing a puff of vapor which facilitate ignition. When anode and cathode circuits are closed with the lamp initially at zero degrees F., there is a delay of about 5 seconds while the cathode warms up. Ignition then occurs with an anode current of about 150 milliamperes; at this time, the discharge glows with a typical pale blue mercury color. The current then falls and the color changes to the more violet color typical of argon for a period of about 40 seconds; thereafter bulb warming becomes appreciable, the current begins to rise again and the pale blue mercury color returns. About one minute after closing the switch, the current rises to a value of about 200 milliamperes and the color of the discharge is that of the ordinary mercury arc.

If, after the above experiment, the lamp is extinguished and recooled to 0 F., it will go through the same cycle again when the switch is reclosed. With one particular lamp, such ignition occurred reliably at 10.8 volts. The explanation for this behavior appears to be that the oxide cathode cools relatively fast when the lamp is turned off and, due to the relatively high mercury vapor pressure Within the heated envelope, mercury condenses thereon. Thus the oxide cathode is ready to vaporize mercury and produce a puff of vapor whenever it is reheated. However, under certain conditions this mode of mercury vapor pulf generation may not function. Thus, if under conditions of low temperature, the starting switch is opened after having been closed for only about 30 seconds, the mercury on the cathode will have been flashed off without being replaced by bombarding mercury ions or by condensation of mercury. If the lamp should then be allowed to cool to 0 F., starting would now require 11.4 volts. The explanation of course is that there is now no mercury condensed on the oxide cathode so that the vapor puff is not produced.

According to one aspect of my invention, there is provided in the lamp of FIG. 1 a special structure serving as a reliable source of an igniting mercury vapor puff and avoiding the drawbacks discussed above. The source proper consists of a material having an affinity for mercury, gold being such a material which amalgamates readily with mercury; other suitable materials are silver and copper; carbon also holds mercury and may be used. The material is placed in such heat relationship to the filament that it is heated to a temperature where it gives up its mercury when the lamp is first turned on but reacquires it during subsequent operation as when the lamp is turned ofi. One suitable arrangement for this purpose consists in keeping the ends of the main tungsten wire cathode filament 6 clear of oxide coating and applying thereto a gold coating, as indicated by the light dotted lining at 20. Alternatively or additionally, gold coating may be applied to the inleads in immediate proximity to the ends of main cathode 6, as indicated by light dotted lining at 21. Gold coating may also be applied to the extremities of auxiliary cathode filament 9, preferably to the left end only, as indicated by the light dotted lining at 22, in order to avoid cooling the more negative right end. When the lamp is turned on, the heating current through main cathode 6 or through auxiliary cathode 9 causes the temperature of the gold coating to rise and mercury atoms are expelled from it, thereby creating the desired puff of mercury vapor. When the lamp is turned off, the gold coating reacquires mercury from the vapor throughout the lamp so that the operation may subsequently be repeated.

FIG. 3 illustrates a variant constituting a preferred embodiment of the invention. Lamp 1a is provided with an equipotential main cathode in the form of an openended cylinder or sleeve 24. The sleeve is metal and desirably thin-walled in order to have a low thermal capacity and heat up rapidly; one way of making it is to wind a tight-pitch coil of tungsten wire which is then sintered to bind all the turns together. Sleeve 24 is provided on the outside with an activating coating of alkaline earth oxides (shown by -lig-ht s-tippling), and on the inside with an insulating coating, suitably an aluminum oxide glaze (shown by light dotted lining where part of the sleeve is broken away). Cathode sleeve 24 is supported from inlead 7 by means of a ring clamp 25 encircling the end remote from the inlead and a strap 26 spot-welded thereto. A bare tungsten wire coil 27 extends through the sleeve and has its ends clamped or cold-Welded to inleads 7 and 10. Coil 27 is a heater for the sleeve and at the same time serves as a high work function auxiliary cathode which reduces the ignition voltage by emitting a small percentage of high potential energy electrons. The portion of coil 27 extending beyond sleeve 24 close to inlead 7 emits electrons in a favorable position for producing ionization. At the same time, the place of attachment of ring clamp 25 permits the end of the sleeve closer to inlead 7 to heat up rapidly, as is desirable for rapid ignition.

In accordance with the present invention, a decided improvement in the low temperature starting ability of the lamp of FIG. 3 is achieved by providing an auxiliary gold coated conductor 28, suitably of nichrome resistance wire which has a lower positive temperature coefiicient of resistance than tungsten. Conductor 28 extends between inlead 10 and an additional inlead 29, parallel to sleeve 24 in the space between the sleeve and the bight of anode 4. The cross section of conductor 28 is such that flow of heater current therethrough raises its temperature to about 135 C. This temperature is intermediate between that of the cathode and that of the envelope and is sufficiently high to vaporize the mercury absorbed in the gold coating. Conductor 23 thus serves as a mercury vapor puff source.

FIG. 4 illustrates a suitable circuit for operating the lamp of FIG. 3. As illustrated, inlead 10 becomes a dummy lead and no longer conducts current from outside the envelope; current is conducted by lead 7 through heater 27, through conductor 28 serving as a mercury puff source, and out through additional inlead 29; resistor 30 limits the heater current flow. The circuit is in other respects similar to that of FIG. 2 previously described.

Conductor 28 as a mercury puff source can become loaded with mercury in any of the three following ways:

(1) By standing in contact with the mercury vapor normally present during operation of the lamp.

(2) By cooling at a faster rate than the lamp envelope when the lamp is turned off and thus picking up mercury during the cooling down process.

(3) By being maintained at a negative potential during the discharge and so taking up mercury in the form of positive ions.

The size and general characteristics of the mercury pufl source may be determined on the basis of the following considerations wherein mercury loading of the source is assumed to occur by method 1) only; additional mercury contributed by methods (2) and (3) is considered to provide a margin of safety. I have established experimentally that the pressure of mercury vapor obtainable with a bulb at room temperature sufiices to fire a lamp in 0.2 second, and from this the quantity of mercury required can be calculated approximately. The calculation may be based on the assumption of vapor emission corresponding to an envelope temperature of C. This envelope temperature is somewhat higher than required for ignition and allows a margin for the fact that the source is localized, rather than filling the anode-cathode space.

If the source were pure mercury at 40 C., it would vaporize 842x10 atoms per cm. of surface per second, corresponding to a saturated vapor pressure of 6 microns. Continuing this vaporization rate for 0.2 second, a total of 1.68 l0 atoms per cm. of surface are evaporated. Assuming a gold plating thickness or depth of 10- cm. and an amalgam formed to that depth, the source so constituted must contain a mol fraction of approximately 0.00413 of mercury. Based on Raoults Law which says that the vapor pressure of mercury should be proportional to its mol fraction at any temperature, the temperature required at the source to deliver mercury at the calculated rate from the calculated mol fraction may be determined. Leaving out a second-order temperature term, the saturated vapor'pressure over'puremercury at the temperature of the source is equal to the 6 microns quoted above, divided by the mol fraction, as follows:

For a vapor pressure of 1.5 mm. over pure mercury, a temperature of C. is required; this, then, is the temperature needed at the amalgam source for therequired rate of vaporization. Therefore gold-plated conductor '28, as a result of resistive heating and heat gained by radiation or conduction from the cathode, must achieve during normal operation a temperature of approximately 135 C. In its physical dimensions, the mercury puif source may be a wire 28, as illustrated in FIG. 3, of the order of 10 to 15 mils in diameter, suitably of nichrome and plated with gold to a depth of about 10 cm., and approximately 1 cm. long; alternatively a band approximately 3 mils thick and 15 mils wide plated with gold to the same depth may be used.

In the normal operation of the lamp, the mercury puif source runs at about 135C. and comes to equilibrium with the mercury vapor pressure within the bulb. With a bulb temperature of 40 C., the gold coating on the source takes up a mol fraction of about 0.00413 of mercury. Due to the fact that the source is negative in potential relative to the space around it, it collects mercury ions and thus takes up slightly more mercury than was indicated earlier. Also on turning oif the lamp, the source cools at a faster rate than the envelope and thereby picks up a little more additional mercury. In turning on the lamp, the source heats up to about 135 C. while the envelope is still cold. Thus the vapor pressure of the amalgam is much higher at this moment than that existing in the bulb; the source thus releases mercury vapor for ignition of the arc.

By utilizing an operating circuit which cuts down the cathode heating current after the lamp has ignited, the mercury puff source maybe caused to rise to a temperature higher than the calculated 135 C. at starting. The circuit illustrated in FIG. 4 does this and also protects the cathodes from battery overvoltages. When are current flows, the resistance of ballast lamp'17 increases and a relatively large voltage drop occurs across it. Since the cathode heating current is governed by the applied voltage effective after deducting from the battery voltage the voltage drop across the ballast lamp, the cathode heating current accordingly decreases and the temperature of mercury puff source 28 drops after the initial rise. The net result of the foregoing is that the mercury puff source releases a larger quantity of mercury vapor at starting and picks up a larger quantity during normal operation, both actions being in a direction to favor low temperature ignition.

While certain specific embodiments of the invention have been illustrated and described, they are intended as illustrative and not as limitative of the invention. Various modifications will readily occur to those skilled in the art, and it is intended by the appended claims to cover any such as fall within the true spirit and scope of the invention.

What I claim as new and desire to secure by Letters Patents of the United States is:

1. A thermionic are electric discharge lamp of the cathode glow type having an operating voltage of the order of magnitude of the ionizing potential of mercury, comprising an envelope containing a buffer gas at a low pressure and a small quantity of mercury and having sealed therein an anode and a thermionic activated cathode heated by current flow at starting, and a mercury vapor puff source within said envelope comprising a coating of a material having an affinity for mercury, on a portion of said cathode so that a rapid rise in temperature of said material occurs upon flow of heating current through said cathode and causes said material to release a puff of mer- =1500,u, or 1.5 mm.

cury vapor into said envelope, thereby lowering the starting voltage of said lamp under low ambient temperatures.

2. A thermionic arc electric discharge lamp of the cathodic glow type having an operating voltage of the order of magnitude of the ionizing potential of mercury comprising an envelope containing an inert starting gas at a pressure of a few millimeters and a small quantity of mercury, and having sealed therein an anode, a main thermionic filamentary low work function cathode for supporting the major part of the electron discharge current and an auxiliary thermionic high work function cathode of tungsten wire operable at the most negative potential in the lamp in order to inject high speed electrons into the discharge space to facilitate starting, and a mercury vapor puff source comprising a material having an afiinity for mercury and located proximate to said cathode so as to be in close thermal coupling therewith and serving to provide a puff of mercury vapor to facilitate starting of said lamp under low ambient temperatures.

3. A thermionic are electric discharge lamp of the cathodic glow type having an operating voltage of the order of magnitude of the ionizing potential of mercury, comprising an envelope containing a buffer gas at a low pressure and a small quantity of mercury and having sealed therein an anode and a thermionic activated cathode heated by current flow at starting, said cathode comprising a coiled tungsten wire supported on inleads and having a central portion coated with activating material and having on its ends close to the supporting inleads a coating of a material having an aflinity for mercury, whereby the rise in temperature of said material occurring upon flow of heating current through said cathode causes it to release a puff of mercury into said envelope and thereby lower the starting voltage of said lamp under low ambient temperatures.

4. A lamp as defined in claim 3 wherein said coating of material having an aifinity for mercury consists of a layer of gold.

5. A thermionic are electric discharge lamp of the cathodic glow type having an operating voltage of the order of magnitude of the ionizing potential of mercury comprising an envelope containing a buffer gas at a low pressure and a small quantity of mercury and having sealed therein an anode and a thermionic activated cathode, and a mercury vapor puff source within said envelope comprising a resistive conductor of nichrome wire plated with gold and connected in series with said cathode so as to be heated by current flow therethrough and achieve at starting a temperature intermediate between that of said envelope and that of said cathode in order to release a puff of mercury vapor into said envelope to lower the starting voltage under low ambient temperatures.

6. A thermionic arc electric discharge lamp of the cathodic glow type having an operating voltage of the order of the ionizing potential of mercury comprising an envelope containing a buffer gas at a low pressure and a small quantity of mercury, and having sealed therein an anode, a unipotential sleeve cathode coated with activating material to achieve a low work function and an unactivated tungsten wire heater extending therethrough for heating same and serving as an auxiliary high work function cathode for injecting high speed electrons into the discharge space to facilitate starting, and a resistive conductor plated with a material having an aflinity for mercury connected in series with said heater and extending through the region between said anode and cathode and serving as a mercury vapor puif source upon flow of heater current whereby to lower the starting voltage of said lamp under low ambient temperatures.

7. A thermionic are electric discharge lamp of the cathodic glow type having an operating voltage of the order of the ironizing potential of mercury comprising an envelope containing argon with a minor proportion of xenon at low pressure and a small quantity of mercury, and having sealed therein an anode, a unipotential sleeve cathode coated with activating material to achieve a low work function and an unactivated tungsten wire heater extending therethrough for heating same and serving as an auxiliary high work function cathode for injecting high speed electrons into the discharge space to facilitate starting and a gold plated resistive conductor connected in series with said heater and extending through the region between said anode and cathode and serving as a mercury vapor pulf source upon flow of heating current whereby to lower the starting voltage of said lamp under low ambient temperatures.

References Cited by the Examiner UNITED STATES PATENTS 2,047,018 7/1936 Fairbrother et al. 3l5108 2,43 8,181 3/1948 Morehead et al 3l3225 2,832,912 4/1958 Lake 3 l3109 DAVID J. GALVIN, Primary Examiner. 

3. A THERMIONIC ARC ELECTRIC DISCHARGE LAMP OF THE CATHODIC GLOW TYPE HAVING AN OPERATING VOLTAGE OF THE ORDER OF MAGNITUDE OF THE IONIZING POTENTIAL OF MERCURY, COMPRISING AN ENVELOPE CONTAINING A BUFFER GAS AT A LOW PRESSURE AND A SMALL QUANTITY OF MERCURY AND HAVING SEALED THEREIN AN AMODE AND A THERMIONIC ACTIVATED CATHODE HEATED BY CURRENT FLOW AT STARTING, SAID CATHODE COMPRISING A COILED TUNGSTEN WIRE SUPPORTED ON INLEASDS AND HAING A CENTRAL PORTION COATED WITH ACTIVATING MATERIAL AND HAVING ON ITS ENDS CLOSE TO THE SUPPORTING INLEADS A COATING OF A MATERIAL HAVING AN AFFINITY FOR MERCURY, WHEREBY THE RISE IN TEMPERATURE OF SAID MATERIAL OCCURRING UPON FLOW OFHEATING CURRENT THROUGH SAID CATHODE CAUSES IT TO RELEASE A PUFF OF MERCURY INTO SAID ENFELOPE AND THEREBY LOWER THE STARTING VOLTAGE OF SAID LAMP UNDER LOW AMBIENT TEMPERATURES. 